Then write the letter yourself.
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no lawyer required in Canada. You are free to walk away. You asked for an estimate. They decided to ignore your request and did some work, that you did not ask for. They made a mistake. They own it. It is their problem, not yours.Sounds like you need a lawyer friend to craft a letter that says, in effect, "you repaired by dive computer without prior authorization to do so, in conflict with existing statutes, and in good faith I have offered you $300 for your effort, which you have rejected. This matter is now closed. Please address any further correspondence to my attorney, XXX."